National Park Service workers were seen emptying jugs labelled “12% hydrogen peroxide” into the visibly green water ​of the nearly century-old basin that stretches between the Lincoln ​Memorial and the Washington Monument on the National Mall.

A spokesperson ⁠for the Interior Department said the pool is being treated with “high-tech nanobubble ​ozone technology” that kills algae, pathogens including E. coli, and other contaminants.

​The department said hydrogen peroxide is also being used, saying it’s “a milder treatment than chlorine” that is used in spas and natural swimming pools.

Christopher Gobler, a professor ​at Stony Brook University in New York who has studied harmful algal ​blooms for more than 30 years, said hydrogen peroxide is a common short-term treatment ‌for ⁠algae that is not likely to have major health effects on the ducks and geese that frequent the pool.

“It’s most harmful to animals that are 100% in the water,” Gobler said, adding the chemical can create ​low oxygen conditions ​that can be ⁠lethal to fish.

It might not be a long-term fix, though, he warned.

“There’s always going to be a rebound ​of the algae,” Gobler added. “Hopefully it works as a ​quick ⁠fix, but these algae do tend to rebound.”

The reflecting pool was refilled with water starting on June 5 as part of a renovation project directed ⁠by ​Trump that involved draining, cleaning and resurfacing ​it.

The bottom of the pool was repainted in a colour Trump calls “American flag blue.”